An input/output (IPO) circuit within a particular device may serve as an electrical interface between the device's internal circuitry and the external circuitry belonging to other devices. The I/O circuits may serve to exchange (transmit and/or receive) voltage signals between the internal and external circuitry. Such IPO circuits may also be used to provide electrical isolation between the internal circuitry and the external circuitry, and may be especially useful when the internal circuitry operates at different voltages than the external circuitry. For example, the internal circuitry may include an integrated circuit core which may function at lower voltages, and the external circuitry may be part of a peripheral device which may function at higher voltages. In this situation, the I/O circuit can serve to protect the circuit core from the higher voltages generated by the peripheral device.
The I/O circuit may operate using a positive voltage supply and a negative voltage supply. In general, these supplies may represent the most positive and most negative voltages generated within the I/O circuit itself. In some situations, the negative supply may simply be at ground potential (i.e., the negative supply may be designated as zero volts).
When in output mode, the I/O circuit may be providing output signals to the external circuit. The voltage values of the output signals may be bounded by the voltages produced by the I/O circuit's voltage supplies. During input mode, the I/O circuit is receiving input signals from the external circuit. For conventional I/O circuits, the input signals should be limited to the values capable of being produced by the voltage supplies for reliable operation. If the input voltages are not so limited (hereinafter defined as “exceeding voltage input signals”), components within the I/O circuit may become electrically over-stressed. Specifically, exceeding voltage input signals may lead to undesirable leakage currents through transistors used in the I/O circuit, and can further lead to a variety of breakdown phenomena, including oxide breakdown, hot carrier injection, negative bias temperature instability, and junction breakdown.
In some situations, the IPO circuit may not be able to avoid receiving exceeding voltage input signals as the input signal's amplitude is controlled by the external circuit. In order for a device to interface to a wide variety of peripherals, it may be desirable to design I/O circuits with the capability to receive exceeding voltages (within practical limits) without any detrimental effects. I/O circuits having this capability may be defined as being “voltage tolerant.”
Accordingly, there is a need for voltage tolerant IPO circuits which may accept exceeding voltage input signals from external circuits which minimize leakage currents and improve reliability, performance, and power efficiency.